Chicago Fire Department spokeswoman Meg Ahlheim said crews checked carbon monoxide levels in the building but didn't find evidence of a leak. The Tribune reports fire crews returned once again after a 74-year-old in the building was found unresponsive around 3:45 p.m. The second pass-through found a positive reading for "a low level of carbon monoxide" near a boiler in the basemen t— though there were still no indications of a leak in the residential units.

NBC said five other children were taken to the hospital for observation.

Peoples Gas was called to fix the boiler, and a spokeswoman told the Sun-Times the company was "supporting" fire officials in the investigation.

An official cause of death won't be determined until the Cook County Medical Examiner's office completes autopsies on Monday.